Where do Atlantic Tropical Cyclones Form?

The official start to the 2015 Atlantic Hurricane Season is fast approaching on June 1st. With that in mind, a question I want to answer today is: Where do tropical storms and hurricanes form?

To answer this I turned to the National Hurricane Center’s Best Track Data in their HURDAT2 dataset The hurricane seasons for the period 1950-2014, and only tropical and subtropical cyclones with top 1-minute sustained winds greater than 34kts were included. The HURDAT2 dataset contains 6-hourly data for Atlantic Tropical cyclones. Once it was determined that the tropical cyclone had fit the above criteria (1950-2014, at least TS,) its genesis coordinates were recorded (the latitude and longitude of its first entry as a tropical or subtropical cyclone).

These coordinate locations were split up into cyclone groups based on their peak intensity as follows:

Using an R spatial statistics package {spatstat} the spatial density of the genesis locations was calculated and plotted on a map of the Atlantic Basin. The resolution for these plots is roughly 0.664 x 0.291 degrees or about 76 km for the diagonal across each pixel.

Some interesting patterns are immediately evident in the distribution of genesis location density. For ‘All Tropical Cyclones,’ there are two main regions:

1. The open Tropical Atlantic known as the ‘Main Development Region – MDR’ (the area between the West Coast of Africa and the Caribbean)

2. The Southwest Atlantic (the area north of the Bahamas, the northwest Caribbean, and the Gulf of Mexico)

In region 1, the main source of tropical cyclones is likely African Easterly Waves, inverted troughs that move from east to west into the Atlantic associated with disturbances in the West African Monsoon. In region 2, some of these African Easterly Waves don’t encounter an environment favorable for development until they get steered into or North of the Caribbean where they have another chance to develop. Less frequently, midlatitude weather systems (extratropical cyclones) get steered southward into the subtropics and tropics where they encounter relatively weak steering flow and begin to transition into subtropical or tropical cyclones over the warm waters. This is more common north of the Bahamas. A further mechanism for development, particularly in the Northwestern Caribbean and Gulf of Mexico, is related to the monsoon trough over Central America. This trough, present mainly during the summer months, can be displaced northward into the Northwest Caribbean or Gulf of Mexico where, if it persists under favorable conditions, it can organize into a tropical cyclone. The number of cyclones that form in region 2 is similar to that in region 1.

For those tropical cyclones that did not intensify to become hurricanes, ‘Just Tropical Storms,’ most form in the Gulf of Mexico or north of the Bahamas. This area takes up most of region 2 from above. This is likely because cyclones that form in the Gulf of Mexico are so close to land that they either make landfall and dissipate, or interact with dry air that disrupts their circulations.

For the cyclones north of the Bahamas, this is also a two-pronged possible explanation; cyclones that form here are typically quickly pulled poleward into the midlatitudes where they encounter wind shear and cooler waters, or they are formed from transitioning extratropical cyclones. The midlatitudes are a hostile place for tropical cyclones because of the strong wind shear associated with the jet stream, the cooler waters further north, and dry air. Dry air and wind shear from the midlatitudes is often nearby transitioning extratropical cyclones and this often slows the transition and limits intensification.

Region 2 is also active through the entire season whereas region 1 is active mainly during the peak of the season. Less than ideal early and late season conditions also likely play a role in limiting the intensity of cyclones here.

Probably for similar reasons, minimal-moderate hurricanes form most frequently in the Northwest Caribbean as seen on ‘Minimal Hurricanes Only.’ Here, they quickly interact with land limiting their intensification. Meanwhile, on the ‘Major Hurricanes Only’ plot, we see that the most common formation area is off the west coast of Africa. Cyclones that form in this region are notorious for their longevity and intensity and are informally known as “Cape Verde-type” Hurricanes. This region not only becomes most active during the peak of the season when the tropical Atlantic is most favorable for tropical cyclone development, but with a typical track towards the west-northwest, there is no land to interact with for over a thousand miles – plenty of open ocean to intensify over.

A more subtle pattern that becomes most evident in the ‘All Hurricanes’ plot are the two maximum of formation frequency west of the Cape Verde islands. This could be due to diurnal variations in convective intensity that help to organize the African Easterly Waves (ie. showers and thunderstorms flare up around the disturbance at around the same time each day – think afternoon thunderstorms.) Since each of these waves move westward at similar speeds, the maximum near the Cape Verde island could be related to the first diurnal maximum in convection, and the maximum in the central tropical Atlantic (further to the west) could be related to the second diurnal maximum. (ie. the disturbance becomes a tropical cyclones after one day over water or after two days.)

Wind shear and stronger trade winds in the Eastern Caribbean limit tropical cyclone formation. The wind shear disrupts the organization of deep convection – a key element of tropical cyclones. Similarly, strong trade winds keep the initial disturbance for forming a closed circulation which prevents the tropical cyclone from sustaining the deep convection it needs. Overall, conditions are not as favorable for tropical cyclone formation in the Eastern Caribbean despite the adequately warm waters.

It should be noted that while more major hurricanes are born in the Cape Verde region than anywhere else in the Atlantic, cyclones that form here do not necessarily impact any land and can therefore go unnoticed. The cyclones that form in the Southwestern Atlantic are much more likely to impact land, particularly when they form in the Caribbean and Gulf of Mexico, but they less frequently become major hurricanes. The most intense hurricane on record, Hurricane Wilma (Oct, 2005), formed in the Northwest Caribbean. Bermuda has also been impacted by major hurricanes originating from both of the active regions of the Atlantic. Hurricane Fabian (Aug-Sep, 2003) formed in the Main Development Region, while the Havanna-Bermuda Hurricane formed in the Northwest Caribbean (Oct, 1926.)

Where do landfalling cyclones and cyclones that impact Bermuda typically form? See a follow-up post for the answer to this!